LibWeb: Guarantee used values exist wherever layout reads them

[kalenikaliaksandr]

LayoutState::try_used_values let callers probe whether a box has per-pass used values, conflating "give me the used values" with "did this box participate in layout this pass". The Option-ness was C++ heritage from the retired engine's nullable LayoutState::get(); against pass-local state, every layout-time read site can know the answer statically. Restructure so used values are guaranteed to exist at every read, then drop try_used_values and the per-context try_used_pointer wrappers.

[coderabbitai]

📝 Walkthrough

Walkthrough

The layout engine replaces optional used-value lookups with direct access, adds shared node participation predicates, updates inline geometry state, passes content inline size into line-data emission, and centralizes table row-group classification and participation logic.

Changes

Layout used-values and participation

Layer / File(s)	Summary
Layout facts and access contracts Libraries/LibWeb/Rust/src/css/display.rs, Libraries/LibWeb/Rust/src/layout/layout_state.rs	The code adds shared predicates for table row groups, flow participants, and out-of-flow nodes. Optional used-value access is removed from LayoutState.
Used-value initialization and block sizing Libraries/LibWeb/Rust/src/layout/abspos_engine.rs, Libraries/LibWeb/Rust/src/layout/block_formatting_context.rs, Libraries/LibWeb/Rust/src/layout/formatting_context.rs, Libraries/LibWeb/Rust/src/layout/grid_formatting_context.rs	Absolute-positioned, block, grid, and generic formatting contexts directly retrieve or create used values. Block sizing and baseline selection use flow-participant filtering.
Inline geometry and line-data flow Libraries/LibWeb/Rust/src/layout/inline_formatting_context.rs, Libraries/LibWeb/Rust/src/layout/inline_level_iterator.rs, Libraries/LibWeb/Rust/src/layout/layout_state.rs	Inline layout consolidates interrupting-block geometry, requires used values for geometry calculations, excludes out-of-flow fragmented ancestors, and passes content inline size to push_line_data.
Table run participation and row groups Libraries/LibWeb/Rust/src/layout/table_formatting_context.rs	Table layout adds child participation logic and uses the unified row-group predicate for participant seeding, positioning, and baseline calculation.

Estimated code review effort: 4 (Complex) | ~45 minutes

Possibly related PRs

• LadybirdBrowser/ladybird#10971: Overlaps the same formatting-context, used-value, and root-sizing paths.
• LadybirdBrowser/ladybird#10946: Shares FfiDisplay row-group predicates and table layout handling.
• LadybirdBrowser/ladybird#10968: Shares inline push_line_data and relative-position used-value changes.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description directly explains the removal of optional used-value access and the restructuring of guaranteed pass-local used values.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

🧹 Nitpick comments (1)

Libraries/LibWeb/Rust/src/layout/layout_state.rs (1)

505-512: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Document that is_flow_layout_participant still includes floating boxes.

is_flow_layout_participant excludes absolutely positioned boxes, list-item markers, and SVG mask/clip boxes, but not floating boxes. Callers in derive_baselines and automatic_block_size_for_bfc_root add a separate is_floating() check next to this predicate. Add a short doc comment on is_flow_layout_participant that states floats still pass this check, so future callers do not assume it covers floats.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@Libraries/LibWeb/Rust/src/layout/layout_state.rs` around lines 505 - 512, Add
a short documentation comment above is_flow_layout_participant stating that
floating boxes still satisfy this predicate and must be checked separately with
is_floating(). Leave the predicate logic and callers such as derive_baselines
and automatic_block_size_for_bfc_root unchanged.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@Libraries/LibWeb/Rust/src/layout/layout_state.rs`:
- Around line 505-512: Add a short documentation comment above
is_flow_layout_participant stating that floating boxes still satisfy this
predicate and must be checked separately with is_floating(). Leave the predicate
logic and callers such as derive_baselines and automatic_block_size_for_bfc_root
unchanged.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: e7b7b22f-bba5-4e28-a9a1-c3ccd728834c

📥 Commits

Reviewing files that changed from the base of the PR and between 692d8b4 and dc0b5c5.

📒 Files selected for processing (9)

• Libraries/LibWeb/Rust/src/css/display.rs
• Libraries/LibWeb/Rust/src/layout/abspos_engine.rs
• Libraries/LibWeb/Rust/src/layout/block_formatting_context.rs
• Libraries/LibWeb/Rust/src/layout/formatting_context.rs
• Libraries/LibWeb/Rust/src/layout/grid_formatting_context.rs
• Libraries/LibWeb/Rust/src/layout/inline_formatting_context.rs
• Libraries/LibWeb/Rust/src/layout/inline_level_iterator.rs
• Libraries/LibWeb/Rust/src/layout/layout_state.rs
• Libraries/LibWeb/Rust/src/layout/table_formatting_context.rs